1. Field of the Invention
The present invention relates generally to vertical aperature correction of video signals, and more particularly is directed to a circuit for applying a vertical aperture correction to the luminance signal in a color television camera system.
2. Description of the Prior Art
Color television cameras of the photo-conductive type, such as vidicon, single-tube color television cameras, do not exhibit the picture sharpness which is characteristic of "image orthicon" black and white TV cameras. The vidicon, single-tube color television cameras lack the "edge sensitivity" characteristic of the image orthicon cameras, both because of leakage at the target electrode of the vidicon, and because of the finite size of the scanning electron beam. In a color television camera, the loss of sharpness is compounded because it is necessary to utilize a comparatively narrow contrast range in order to get a good color signal.
Arrangements have been suggested, for instance in U.S. Pat. No. 2,957,042 (Gibson et al.), for compensating for the loss of sharpness in the direction perpendicular to the direction of line scanning. Since the above described loss of detail is analogous to aperture aberration in a photo optical system, and because the direction perpendicular to line scanning is usually vertical, such an arrangement is frequently termed "a vertical aperture compensation system."
Typically, the prior art vertical aperture compensation system delays the luminance signal for a time period corresponding to one line or horizontal scanning interval, and then combines the delayed luminance signal with the undelayed luminance signal. In this way, there is provided a vertical aperture correction signal which emphasizes the difference in luminance from line to line. The vertical aperture correction signal for each line scanning interval is then combined with a respective horizontal line interval of the luminance signal. The resulting combined signal accentuates changes in contrast from line to line, thereby providing a sharper video picture.
Nevertheless, the luminance signal of a color video signal produced by a color television camera, even with a vertical aperture correction circuit, can include undesirable level changes from one horizontal line scanning interval to another. Such changes in luminance level may originate in a color television camera of the index type, or may arise from other causes. In a typical color television camera of the index type, such as that described in U.S. Pat. No. 3,784,737, having a common assignee herewith, a predetermined alternating potential or bias distribution is established on the photoelectric conversion layer for providing an alternating index signal superposed on the video output for use in separating individual color signals from the chrominance component. However, this alternating potential or bias causes the luminance signal in consecutive line scanning intervals within each field to take on levels that vary from one line scanning interval to the next. Such change in level from line to line results in a "line crawling" effect in the luminance signal.
Prior art vertical aperture correction circuits do not provide a vertical aperture correction signal which is free from the line crawling effect. In fact, prior art vertical aperture correction circuits often combine the luminance information in consecutive horizontal scanning intervals in such a way as to add together the undesirable level changes, thereby accentuating the line crawling effect, while attempting to enhance the vertical detail in the television picture.